Polls have shown Tisei leading the embattled Democrat, and he seems pretty confident of victory: In these last days of the campaign, he’s spending money on a TV spot offering a serene beach scene as a respite from the bickering.

Anything can happen between here and Tuesday. But for now, let’s assume Tisei goes to Washington. How does a pro-choice, openly gay man who refuses to sign Grover Norquist’s destructive antitax pledge operate in a party that is utterly not on his wavelength? How does a guy who is willing to cross the aisle do in a party that has made obstruction central to its agenda?

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“I don’t have any problem standing up for myself and my beliefs,” he says. He reckons the reason the country is in so much trouble right now is that “people aren’t willing to sit down and work together and get something done.” Tisei, who says his party has lost its way on civil rights, believes he could help shift Republicans on gay rights — not only by arguing for them, but also by “the mere fact of being there.”

It’s perfectly plausible that Tisei would play that lonely role on social issues in his party. But it’s a good deal harder to imagine him as an iconoclast on the fiscal issues central to Washington’s dysfunction.

This is partly because there’s not as much distance between Tisei and his party’s dominant right wing on fiscal matters. For example, though he acknowledges that the federal stimulus helped Massachusetts out of a jam, he parrots the willfully blind view that it did nothing for the private sector, and says he would have voted against it. He calls the Buffett Rule, which would have ensured that the superrich pay at least 30 percent tax, “class warfare.”

But there is also apostasy: Tisei is in favor of raising revenue by closing unfair tax loopholes. And he is willing to say that “wealthy people probably don’t pay enough in taxes because most of the loopholes and deductions put in place over the years have been put in place for them.” Fancy that!

If Tisei thinks that kind of thing is going to fly in today’s Washington, he holds a far rosier view of Republican bigs than Thomas Mann, coauthor of “It’s Even Worse Than It Looks,” which blames an extremist GOP for congressional gridlock.

“There have been moderates in the last two Congresses,” says Mann, a senior fellow at the center-left Brookings Institution, who co-wrote the book with Norman ­Ornstein of the right-of-center American Enterprise Institute. “But all of them felt obliged to maintain a position of absolute opposition [to administration policies], and no one spoke up.” If moderates don’t fall in line with what Mann calls the party’s “know-nothing-ism on economic policy,” they are “completely marginalized.”

He cites the example of Speaker John Boehner, who told freshman congressmen at the beginning of the 112th Congress that he would not play politics with the debt ceiling, then ended up scrambling to get out in front of Eric Cantor and others who insisted on doing exactly that.

If Boehner couldn’t stand up to these guys, how could Tisei? If, as Mann points out, today’s GOP no longer has room for “George W. Bush, George H.W. Bush, and certainly for Ronald Reagan’s pragmatism,” how will Tisei find his voice — particularly if Mitt Romney becomes president, further empowering the hard-liners?

By joining with like-minded members to advocate a more reasonable way, Tisei says. He points to the fact that dozens of Republican challengers and incumbents have refused to take the antitax pledge this year, making them future allies.

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